Đáp án`+`Giải thích các bước giải:

`(1 + 1/(1 . 3)) . (1 + 1/(2 . 4)) ... (1 + 1/(2019 . 2021))`

`= (2 . 2)/(1 . 3) . (3 . 3)/(2 . 4) ... (2020 . 2020)/(2019 . 2021)`

`= ((2 . 3 ... 2020) . (2 . 3 ... 2020))/((1 . 2 ... 2019) . (3 . 4 ... 2021))`

`= (2 . 2020)/(1 . 2021)`

`= 4040/2021`

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{\text{d}}$$\color{#53008}{\text{M}}$$\color{#6b28ac}
{\text{a}}$$\color{#8c4ebd}{\text{t}}$$\color{#B592D6}
{\text{h}}$$\color{#ceaedf}{\text{s}}$